In order to remove dry dust from industrial waste gases, it is known to use fabric filters; see, for example, Federal Republic of Germany OS 26 33 773 or else the book "Dust Removal by Bag Filters and Pocket Filters" by F. Loffler, published by Vieweg-Verlag, 1984, pages 236, et seq. These fabric filters are generally constructed in the shape of a bag and pulled loosely over a supporting grid which they surround with a slight spacing. The fabric surfaces are generally impinged from the outside by the crude gas which is to be purified so that a filter cake of dust particles deposits on the outer surface and the pure gas is lead away through the inside of the bag. In order to assure the filtration function, the fabric must be cleaned at short intervals. This is done by stretching the fabric in the direction opposite the direction of flow of the gas or perpendicular to the direction of flow. Most frequently this is achieved by the fabric being impacted by a gas, for instance compressed air, which is introduced in pulse-like manner by a nozzle into the inside of the bag filter. This method is disadvantageous in that the pressurized cleaning air flows through the filter cloth and partially breaks up the filter cake and the particles of dust detached thereby are deposited on adjacent filter bags. This effect increases with increasing length of the bag.
It is thus an object of the present invention to provide a method and apparatus for breaking up of the filter cake during the cleaning of fabric filters so that as few components of the filter cake as possible are brought into suspended condition, and therefore, the largest possible amount of the filter cake can be removed as a coherent body.
The present invention is directed to a bag filter assembly including a housing, a supporting grid inside the housing, and bag filter having an outer circumference and being arranged on the supporting grid and sufficiently permeable for permitting a gas to be cleaned to pass therethrough from a crude-gas side outside the supporting grid to a clean-gas side inside the supporting grid, a gas injector for directing a stream of gas under pressure from a pressurized gas generating source into the clean-gas side of the supporting grid, the assembly comprising: an expandable bag having an expanded circumference and being disposed at a distance from the filter bag within the clean-gas side of the supporting grid and connected to the gas injector in a pressure-tight manner; the distance of the expandable bag from the bag filter being selected as a function of the pressure produced by the injector and the source of gas and the expandability of the expandable bag so that the expanded circumference of the expandable bag is greater than the outer circumference of the bag filter. Preferably, the expandable bag is formed from an expandable elastic resilient material. As pointed out below, a plurality of weights are attached to and distributed over the expandable bag. Preferably, the weights are vulcanized on the expandable bag. The expandable bag, which can be at least partially gas permeable is preferably connected to a means for generating a vacuum therein, such as, for example, a piston-cylinder unit, which would, at the same time also serve as source for generating the pressurized gas.